Abstract Recent reports showing accumulation of gadolinium (Gd) in the brain following administration of MRI contrast agents have prompted a necessary search for potential causes and associated risks. Currently, there is no literature to explain why the Gd-based contrast agents (GBCAs) deposit in the brain, and whether or not they are causing harm. These contrast agents are used frequently in daily practice, and each dose contributes to the accumulated amount of Gd in the brain. Since there is no correlation between Gd deposited in the brain and days since dose, these deposits do not seem to diminish over time. This issue has provoked responses from patients who are experiencing painful and disorienting symptoms and clinically significant Gd concentrations in their urine and tissues for months to years after receiving a GBCA; this syndrome has been recently described as Gd Deposition Disease (GDD). A concerning report from these patients is a ?brain fog,? which begins early in the disease process. These symptoms are consistent with the known actions of free Gd ions, which inhibit ion-gated channels, and have the potential to induce apoptosis. Unfortunately, there are currently no studies investigating whether the deposition of Gd seen in the brain is intact GBCA or free Gd ions. However, it has long been known that release of Gd from GBCAs in the body is more rapid than measured release in vitro. It is also known that free Gd can bind to the iron-transporting protein transferrin; transferrin is transported into the brain and releases its cargo in areas where Gd accumulates (e.g. globus pallidus and dentate nucleus). This proposal outlines a series of studies which will test the hypothesis that Gd is deposited in the brain by dissociation from GBCA and transport by transferrin. This basic approach to a clinical problem in neuroradiology will be useful for understanding a recently described disease state associated with current imaging practices. The specific aims of this study will be addressed primarily with in vivo models: Aim 1: Determine the speciation of Gd and GBCAs in vivo, and how species in the blood and urine correlate to Gd species in the brain; this aim will be addressed by in vitro binding studies using human and rat plasma, and an in vivo rat study using mild extraction to separate and measure Gd in free or chelated form in tissues; Aim 2: Determine the role of transferrin on Gd deposition in the brain; this will be addressed by using rat models with variable transferrin saturation or nephrogenic systemic fibrosis followed by measurement of brain deposition distribution associated with these variables; Aim 3: Determine how extravasation of GBCAs correlate to whole-body and brain Gd burden; these studies will involve measurement of Gd concentration in the plasma after dosing rats partially intravenously and partially subcutaneously, to determine if there is a correlation between brain deposition and extravasation. Together, these aims will contribute towards improved understanding and management of GDD, specifically brain Gd deposition, and provide clinicians and patients with more insight for appropriate use of GBCAs.